Luxa knows that striking the Overlander across the face will do no one any good. She knows that it will only yield the disapproval of Vikus and the Council, the dislike of the Overlander himself, and her own vindication. And she knows that the former should take precedence and that she should not be such a child when she is a queen, but—
She still strikes him.
She strikes him because she is angry. She strikes him because he brought not only himself but several of her people in jeopardy because of his foolish plans. She strikes him because his actions indirectly drew the blood of people she cares for, and, if she is honest, she strikes him because she was afraid then. Afraid of losing again.
She is afraid that he will pull something like this again and get them all killed.
And striking him evens the score ever so slightly.
It does not work that way. She knows.
But in the moment, she only feels that blinding fury and that suffocating terror, and so she strikes him.